Gas springs may be employed to yieldably clamp a workpiece while operations are performed thereon. Thus a sheet metal part may be held down by gas springs during stamping or forming. The gas springs can be operated with either an external source of gas or can be precharged and operated as a self-contained unit. Self-contained units may be charged with an inert gas at a pressure of 2000 psi, and in use the pressure may increase to 3000-5000 psi. Examples of such gas springs are shown in U.S. Pat. Nos. 4,792,128 and 4,838,527.